Why are the character limits different for services and products?

Google is constantly changing GMB, now formally named Google Business Profile. This character limit they made without official reasoning.

Google is constantly making changes to the elements of GMB. At one point Google allowed 1000 characters in both the service and product fields. However, Google decided to change that and dropped the character count to 300 for the service field (at the time of this writing). In short, there is no specific reason, it is just what Google has decided.